Extension device for a bone anchor, in particular for minimally invasive surgery

ABSTRACT

An extension device for a bone anchor is provided, wherein the bone anchor comprises
         an anchoring section ( 100 ) for anchoring in a bone and   a receiving part ( 200, 200′ ) connected to the anchoring section, the receiving part comprising a central axis (C) and a channel ( 204 ) for receiving a rod, wherein sidewalls of the channel form two free legs ( 205   a,    205   b ),
 
the extension device comprising
 
a first sleeve ( 1, 1′, 1″ ) with a first sleeve axis (c) that is coaxial with the central axis (C), wherein the first sleeve ( 1, 1′, 1″ ) is configured to be detachably coupled to the receiving part ( 200, 200′ ) and
 
a second sleeve ( 2, 2′, 2″, 2″′ ) with a second sleeve axis, wherein the second sleeve is positioned within the first sleeve ( 1, 1′, 1″ ),
 
wherein the first sleeve ( 1, 1′, 1″ ) is configured to be coupled to the receiving part ( 200, 200′ ) such as to inhibit a translational movement of the first sleeve relative to the receiving part along the central axis and
 
wherein the second sleeve ( 2, 2′, 2″, 2″′ ) is configured to be coupled to the receiving part so as to hinder or inhibit a rotational movement of the second sleeve ( 2, 2′, 2″, 2″′ ) relative to the receiving part ( 200, 200′ ) and wherein the second sleeve ( 2, 2′, 2″, 2″′ ) is connected to the first sleeve ( 1, 1′, 1″ ) through a coupling member ( 3 ) that is configured to advance in the first sleeve ( 1, 1′, 1″ ) together with the second sleeve ( 2, 2′, 2″, 2″′ ) in an axial direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present disclosure claims the benefit of U.S. Provisional Patent Application Ser. No. 61/913,777, filed Dec. 9, 2013, the contents of which are hereby incorporated by reference in their entirety, claims the benefit of U.S. Provisional Patent Application Ser. No. 62/013,415, filed Jun. 17, 2014, the contents of which are hereby incorporated by reference in their entirety, and claims priority from European Patent Application EP 13196326.6, filed Dec. 9, 2013, the contents of which are hereby incorporated by reference in their entirety.

BACKGROUND

1. Field of the Invention

The invention relates to an extension device for a bone anchor, in particular for use in minimally invasive surgery (MIS). The invention also relates to a system including such an extension device and a bone anchor, wherein the bone anchor comprises an anchoring section and a receiving part for receiving a rod to couple the rod to the anchoring section. The extension device includes a first sleeve and a second sleeve positioned within the first sleeve and a coupling structure that allows to couple the extension device to the receiving part of the bone anchor and that inhibits a translational and rotational movement of the extension device relative to the receiving part.

2. Description of Related Art

Extension devices, also called head extenders, for pedicle screws for use in minimally invasive surgery are known in the art. For example, U.S. Pat. No. 7,563,264 B2 describes a spinal stabilization system for a minimally invasive procedure wherein detachable sleeves may be coupled to a collar of a bone anchor to allow for formation of the spinal stabilization system through a small skin incision. The detachable sleeve members may allow for alignment of the collars to facilitate insertion of an elongated member in the collars. A coupling system is provided between the sleeve and the collar that inhibits translational movement of the sleeve relative to the collar. In one embodiment, the sleeve may be coupled to a collar of a bone fastener assembly with movable members that may be threaded into threaded openings in the collar.

WO 2013/112689 A2 describes a minimally invasive tower access device comprising an elongated outer sleeve that slidably receives an elongated inner sleeve. A lock nut is used to secure the inner sleeve and outer sleeve in a locked mode.

SUMMARY

It is the object of the invention to provide an improved extension device for a bone anchor, in particular for use in minimally invasive surgery, and a system of such an extension device and a bone anchor that facilitates the surgical steps and improves the safety of the surgical measures such as compression and distraction.

The object is solved by an extension device according to claim 1 and by a system according to claim 16. Further developments are given in the dependent claims.

The extension device is configured to be coupled to the receiving part such that it is locked against translational and rotational movement of the extension device relative to the receiving part. Because translational and rotational movements of the extension device and the receiving part are inhibited, the connection between the extension device and the receiving part is robust. This permits a safe placement of the rod and a set screw for fixing the rod as well as the following surgical steps of adjustment of the spinal stabilization system, such as compression or distraction. These steps can be performed using the extension device attached to the receiving part.

The extension device comprises a first sleeve and a second sleeve positioned within the first sleeve and an interlocking bushing that connects the second sleeve to the first sleeve and allows a controlled axial movement of the second sleeve relative to the first sleeve. After the extension device has been attached to the receiving part, the first sleeve and the second sleeve can be interlocked with each other and with the receiving part by operating the interlocking bushing in one direction. The interlocking connection between the first sleeve and the second sleeve can be released by operating the interlocking bushing in the opposite direction.

A translational movement between the extension device and the receiving part can be inhibited by a form-fit engagement of a circumferential rib that extends at least partially around the longitudinal axis of the device and engages a corresponding circumferential groove. The rib may be provided at the first sleeve and the groove at the receiving part or vice versa.

A rotational movement between the extension device and the receiving part may be inhibited by the engagement of a longitudinal rib that engages a longitudinal groove. The rib may be provided at the first and/or the second sleeve and the groove at the receiving part or vice versa. The form-fit connection of the longitudinal ribs and grooves allows to apply a high torque.

The extension device comprises only few parts which facilitates the assembly and operation of the device.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will become apparent from the description of embodiments using the accompanying drawings. In the drawings:

FIG. 1 shows a perspective exploded view of the extension device according to a first embodiment.

FIG. 2 shows a perspective view of the extension device of FIG. 1 in an assembled state.

FIG. 3 shows a perspective view of a first embodiment of a receiving part of a bone anchor that may form together with the extension device of FIGS. 1 and 2 a first embodiment of a system of an extension device and a bone anchor.

FIG. 4 shows a top view of the receiving part of FIG. 3.

FIG. 5 a shows a cross-sectional view of the receiving part of FIG. 3 along line A-A in FIG. 4.

FIG. 5 b shows an enlarged view of a detail of FIG. 5 a.

FIG. 6 shows a perspective view of a first sleeve of the extension device of FIGS. 1 and 2.

FIG. 7 shows a perspective view from the bottom of a front end portion of the first sleeve of FIG. 6.

FIG. 8 shows a top view of the first sleeve of FIG. 6.

FIG. 9 shows a cross-sectional view of the first sleeve of FIG. 6 along line B-B in FIG. 8.

FIG. 10 shows a cross-sectional view of the first sleeve of FIG. 6 along line D-D in FIG. 9.

FIG. 11 a shows a perspective view of a second sleeve of the extension device of FIGS. 1 and 2.

FIG. 11 b shows an enlarged perspective view of a detail of FIG. 11 a.

FIG. 12 shows a cross-sectional view of the second sleeve according to FIG. 11 a, the cross section taken in a plane containing a longitudinal sleeve axis and extending through the center of the legs of the sleeve.

FIG. 13 shows a perspective view of an interlocking bushing of the extension device of FIGS. 1 and 2.

FIG. 14 shows a cross-sectional view of the interlocking bushing of FIG. 13, the section taken in a plane containing a cylinder axis of the bushing.

FIG. 15 shows a perspective view of a first step of assembling the extension device of FIGS. 1 and 2, wherein the interlocking bushing of FIG. 13 is to be mounted to the rear end section of the second sleeve of FIG. 11 a.

FIG. 16 shows a perspective view of the interlocking bushing and the first sleeve of FIG. 15 mounted together.

FIGS. 17 to 19 a show cross-sectional views of steps of attaching the extension device of the first embodiment to a polyaxial bone anchor.

FIG. 19 b shows an enlarged view of a detail of FIG. 19 a.

FIG. 20 shows a cross-sectional view of the bone anchor with attached extension device according to the first embodiment in a state before the extension device is rotationally locked with respect to the bone anchor.

FIG. 21 shows a cross-sectional view of an upper portion of the extension device in the state shown in FIG. 20.

FIG. 22 shows an enlarged cross-sectional view of the bone anchor extension device in a state in that the extension device is rotationally locked with respect to the bone anchor.

FIG. 23 shows a cross-sectional view of the upper portion of the extension device in the state shown in FIG. 22.

FIG. 24 shows a perspective view of a receiving part according to a second embodiment that forms together with an extension device according to a second embodiment a second embodiment of a system of an extension device and a bone anchor.

FIG. 25 shows an enlarged perspective view of a front end portion of first sleeve of the extension device according to the second embodiment.

FIG. 26 shows a perspective view of the front end portion of the extension device according to the second embodiment with a bone anchor having a receiving part according to the second embodiment shown in FIG. 24.

FIG. 27 shows an enlarged perspective view of a front end portion of a second sleeve of the extension device according to the second embodiment.

FIGS. 28 to 30 a show cross-sectional views of steps of attaching the extension device according to FIGS. 25 to 27 to a polyaxial bone anchor.

FIG. 30 b shows an enlarged view of a detail of FIG. 30 a.

FIG. 31 shows an enlarged perspective view of a modified embodiment of a second sleeve of an extension device.

FIG. 32 shows a perspective view of a front end portion of a still further modified embodiment of the second sleeve.

FIG. 33 shows a perspective view of a section of the spinal column with inserted pedicle screws and attached extension devices according to the invention.

FIG. 34 shows a perspective view of a receiving part according to a third embodiment that forms together with an extension device according to a third embodiment a third embodiment of a system of an extension device and a bone anchor.

FIG. 35 shows an enlarged perspective view of a front end portion of the first sleeve of the extension device according to the third embodiment.

FIG. 36 shows a perspective view of the front end portion of the extension device according to the third embodiment with a bone anchor having a receiving part according to the third embodiment shown in FIG. 34.

FIG. 37 shows a top view of the extension device according to the third embodiment.

FIG. 38 shows an enlarged cross-sectional view of the front end portion of the extension device according to the third embodiment with a bone anchor having a receiving part according to the third embodiment along lines D-D′ in FIG. 37.

DETAILED DESCRIPTION

Referring to FIGS. 1 and 2, an extension device according to a first embodiment comprises a first sleeve 1 that forms an outer sleeve, a second sleeve 2 that forms an inner sleeve and that is positionable within the first sleeve 1 and an interlocking bushing 3. The interlocking bushing 3 is configured to be connected to the second sleeve 2 and is configured to couple the second sleeve 2 to the first sleeve 1 and to permit a controlled motion of the second sleeve 2 relative to the first sleeve 1.

The extension device shown in FIGS. 1 and 2 is configured to be used with a bone anchor that shall be explained first. An example of a such a bone anchor comprises an anchoring element with threaded shank 100 and a spherical segment shaped head 101, as shown in FIGS. 17 to 19 a wherein the anchoring element is pivotably coupled to a receiving part 200. The receiving part 200 is shown more in detail in FIGS. 3 to 5 b. The receiving part 200 typically is a substantially cylindrical part with a first end or top end 200 a and a second or bottom end 200 b, a central axis C, a coaxial bore 201 extending from the top end 200 a to a distance from the bottom end 200 b, a seat 202 for the head 101 of the anchoring element and a lower opening 203 at the bottom end through which the shank of the bone anchoring element can pass through. A substantially U-shaped recess 204 extends from the top end 200 a in the direction of the bottom end 200 b. The recess 204 serves for receiving a rod (not shown). By means of the recess 204 two free legs 205 a, 205 b are formed. In the outer surface of the legs 205 a, 205 b at a distance from the top end 200 a, circumferentially extending grooves 206 a, 206 b are formed that extend from one end of the channel formed by the U-shaped recess 204 to the other end and that are open towards the U-shaped recess 204. An upper side wall 206 a′, 206 b′ and a lower side wall 206 a″, 206 b″ of the groove 206 a, 206 b may be inclined towards the bottom end 200 b.

The receiving part 200 further comprises at the top end 200 a in a free end surface of each of the legs 205 a, 205 b a recess 207 a, 207 b that extends into the legs in a direction coaxial to the central axis (C). The recess 207 a, 207 b is in a top view elongated and closed in a circumferential direction at both end. As can be seen in particular in FIG. 5 b, a cross-section of the recesses 207 a, 207 b is substantially square or may be rectangular. The recesses 207 a, 207 b serve for engagement with a portion of the extension device as described below. Towards the top end 200 a there may be a chamfered section 208 provided at each recess 207 a, 207 b that facilitates engagement of the recess with the corresponding protrusion at the extension device.

In an upper portion of the legs, an internal thread 209 is provided for cooperation with a locking screw (not shown) for fixing the rod.

Referring to FIGS. 6 to 10, the first sleeve 1 of the extension device comprises a longitudinal axis c that is coaxial with the central axis C of the receiving part 200 when the extension device is coupled to the receiving part. The first sleeve 1 further has a front end 1 a and a rear end 1 b.

The first sleeve 1 comprises adjacent to the rear end 1 b a first section 11 with a largest inner diameter compared to other sections of the first sleeve 1. Two coaxial slits 12 a, 12 b extend from the second end 1 b along at least a portion of the first section 11 in a longitudinal direction. One of the sidewalls of each longitudinal slit 12 a, 12 b comprises a wavy structure as can be seen in particular in FIG. 6 for latching with a reduction sleeve (not shown) used for further steps in the surgical procedure, for example for pressing down the rod and inserting a locking screw to fix the rod. Furthermore, an engagement structure 13, for example, a plurality of flat engagement portions, is provided at an outer surface of the first section 11 that form an engagement portion 13 for engagement with a tool. Following the first section 11, the first sleeve 1 comprises a second section 14 with slightly smaller inner diameter than the first section 11 and adjacent thereto a third section 15 with a smaller inner diameter compared to the second section 14 and first section 13. In the third section, an internal thread 16 is provided that is configured to cooperate with the interlocking bushing 3 as described below.

Two elongate substantially U-shaped slits 17 a, 17 b which have a reverse U-shape compared to the slit 204 in the receiving part and that are offset from each other by 180° extend from the front end 1 a toward the rear end 1 b up to a distance from the second section 14 of the first sleeve. The longitudinal slits 17 a, 17 b have a width in circumferential direction that is greater than a diameter of the rod and have the function of permitting the rod to be inserted therethrough. The width of the slits 17 a, 17 b may be substantially the same as the width of the U-shaped recess 204 in the receiving part 200. By means of the slits 17 a, 17 b, the first sleeve 1 comprises two free legs 18 a, 18 b that are intended to cooperate with the free legs 205 a, 205 b of the receiving part 200. The inner surface of the third section 15 of the first sleeve 1 comprises at each of the legs 18 a, 18 b a longitudinally extending substantially cylinder segment-shaped guiding recess 19 a, 19 b into which a portion of the second sleeve can extend to be guided therein. The guiding recesses 19 a, 19 b extend in a longitudinal direction beyond the longitudinal recesses 17 a, 17 b toward the rear end 1 b, as can be seen in FIG. 9.

Each of the legs 18 a, 18 b comprises at a distance from the front end 1 a a projection 120 a, 120 b that extends in a circumferential direction around the longitudinal axis c from one slit 17 a to the opposite slit 17 b. The shape of the circumferential projection 120 a, 120 b is substantially complementary to the shape of the circumferential grooves 206 a, 206 b of the receiving part, as can be seen in particular in FIG. 19 b. An upper surface of the projection 120 a, 120 b that faces toward the rear end 1 b may be inclined toward the rear end to match the complementary inclined upper wall 206 a′, 206 b′ of the groove 206 a, 206 b. A lower surface of the circumferential projection 120 a, 120 b may be perpendicular to the central axis or also inclined toward the rear end 1 b. Between the projection 120 a, 120 b in the end of the recesses 19 a, 19 b an inner surface portion 121 a, 121 b substantially matches an outer surface portion of the receiving part 200 between the grooves 206 a, 206 b and the top end 200 a.

By means of the legs 18 a, 18 b, the first sleeve is slightly flexible in a radial direction so that the first sleeve 1 can be snapped onto the legs 205 a, 205 b of the receiving part 200.

An outer diameter of the first sleeve 1 may vary between the rear end 1 b and the front end 1 a. The first sleeve 1 may have a greatest outer diameter in the first section 11 followed by one or more successively decreasing outer diameters towards the front end 1 a as shown in FIG. 9. In addition, flattened outer surface portions 122 a, 122 b may be provided at an outer surface of the legs 18 a, 18 b that extend from the front end 1 a toward the rear end 1 b for cooperation with a tool (not shown).

The total length of the first sleeve is such that when the bone anchor is inserted into the bone and the first sleeve is attached to the receiving part 200, the rear end and first section 11 are sufficiently above the operation site when the extension device is used.

Referring now to FIGS. 11 a to 12, the second sleeve 2 comprises a front end 2 a and an opposite rear end 2 b. The second sleeve 2 has a substantially constant outer diameter. Adjacent to the rear end 2 b, the second sleeve 2 comprises a first portion 21 with a circumferentially closed cylinder surface, wherein the first portion 21 is configured to engage the interlocking bushing 3. A plurality of coaxially extending slits 22 that are open end to the rear end 2 b and that extend to a distance from the rear end 2 b are provided to render the first portion 21 flexible in such a way that the first portion 21 can elastically snap onto a portion of the interlocking bushing 3 and is held there by friction. At a first distance from the rear end 2 b a circumferentially extending annular projection 23 projecting inward is provided that cooperates with a corresponding depression or recess at the interlocking bushing 3 to inhibit an inadvertent removal of the interlocking bushing 3 from the second sleeve 2. At a second distance from the rear end 2 b a stop 21 a, for example in the form of an annular shoulder is provided that limits the insertion of the interlocking bushing 3 and provides an abutment for the interlocking bushing 3 when the interlocking bushing is screwed into the first sleeve 1. An outer diameter of the first section 21 of the second sleeve 2 is such that is fits into third section 15 of the first sleeve 1.

Two recesses 24 a, 24 b with substantially rectangular cross section extend from the front end 2 a through the second sleeve 2 up to the first portion 21. The recesses have such a size that two opposite legs 25 a, 25 b are formed that fit into the guiding recesses 19 a, 19 b of the first sleeve 1. The legs 25 a, 25 b have such a length that they extend beyond the upper closed end of the recesses 17 a, 17 b of the first sleeve 1 in a direction toward the rear end 1 b of the first sleeve 1.

The front end 2 a of the second sleeve 2 is comprised of a substantially flat surface portion 26 a, 26 b on each of the legs 25 a, 25 b that is configured to cooperate with a substantially flat surface portion on the top end 200 a of the receiving part, and a projection 27 a, 27 b on each leg, that is configured to cooperate with the recess 207 a, 207 b at the free end of the legs of the receiving part 200. Hence, the projection 27 a, 27 b has a complementary shape to the shape of the recess 207 a, 207 b of the receiving part. As can be seen in FIG. 11 b, a side surface of the projection 27 a that faces toward the longitudinal axis of the second sleeve 2 is flush with the inner surface of the legs 25 a, 25 b. An outer surface of the projections 27 a, 27 b is slightly recessed with respect to an outer surface of the legs 25 a, 25 b. The shape of the projections 27 a, 27 b is substantially arc shaped with rounded edges like the corresponding recess 207 a, 207 b in the receiving part. In a circumferential direction, the projection 27 a, 27 b are arranged substantially in the middle of the legs 25 a, 25 b.

Turning now to FIGS. 13 and 14, the interlocking bushing 3 comprises a front end 3 a and a rear end 3 b and a cylindrical section 31 that is adjacent to the front end 3 a and that has an outer diameter that is substantially the same as an inner diameter of the first section 21 of the second sleeve 2. The size of the inner portion of the first section 21 of the second sleeve 2 and the cylindrical section 31 of the bushing 3 is such that the bushing 3 is held in the second sleeve 2 by friction. Adjacent to the cylindrical section 31 with a smooth outer surface, there is a groove 32 for engaging with the annular projection 23 of the second sleeve 2. Adjacent to the rear end 3 b, a threaded portion 33 with an external thread is provided that is configured to cooperate with the threaded section 16 of the first sleeve 1. An inner diameter of the bushing 3 may be substantially constant and sized such that a locking member for fixing the rod in the receiving part can be inserted therethrough. Adjacent to the rear end 3 b, the inner surface of the bushing 3 comprises an engagement portion 34, for example, a plurality of longitudinally extending grooves and projections for engaging with a tool.

The assembly of the second sleeve 2 and the interlocking bushing 3 is shown in FIGS. 15 and 16. The interlocking bushing is pushed with its cylindrical portion 31 into the first section 21 of the second sleeve 2 until the inwardly projecting annular projection 23 snaps into the groove 32 of the interlocking bushing 3. By means of this, the interlocking bushing 3 is coupled to the second sleeve 2 in such a manner, that a rotational motion of the interlocking bushing 3 relative to the second sleeve 2 is possible. An axial movement of the interlocking bushing 3 relative to the second sleeve 2 is inhibited when the interlocking bushing abuts against the stop 21 a provided in the second sleeve 2. The connection between the second sleeve 2 and the first sleeve 1 via the interlocking bushing is a rigid connection.

The assembly consisting of the second sleeve 2 and the interlocking bushing 2 as shown in FIG. 16 is then inserted into the first sleeve 1 from the rear end 1 b until the threaded portion 33 of the interlocking bushing engages the threaded section 16 of the first sleeve 1. The legs 25 a, 25 b are guided in the guiding recesses 19 a, 19 b in the first sleeve 1. By means of this, the extension device is assembled.

Referring now to FIGS. 17 to 23, the attachment of the extension device to a bone anchor will be described. As shown in FIG. 17, the extension device is moved with the front end 1 a of the first sleeve 1 toward the receiving part 200. Then, as depicted in FIG. 17, the legs 18 a, 18 b of the first sleeve are spread to a certain extent when they touch the top end 200 a of the receiving part 200 and by further downward movement of the extension device, the circumferential projections 120 a, 120 b of the legs 18 a, 18 b of the first sleeve 1 snap into the circumferential grooves 206 a, 206 b that are provided at the receiving part 200. As soon as the projections have engaged the recesses, an axial movement of the first sleeve 1 in a direction away from the receiving part 200 is inhibited.

Then, as shown in FIGS. 19 a and 19 b, the second sleeve 2 is moved relative to the first sleeve 1 toward the receiving part 200 by screwing the interlocking bushing 3 further towards the front end 1 a of the first sleeve 1. When the projections 27 a, 27 b of the second sleeve 2 enter the corresponding recesses 207 a, 207 b at the top end 200 a of the receiving part 200 a rotational movement of the second sleeve 2 relative to the first sleeve 1 and therefore relative to the receiving part 200 is inhibited. Referring to FIGS. 20 and 21, a step of moving the second sleeve 2 relative to the first sleeve 1 is shown. In FIG. 20, the position of the second sleeve 2 relative to the first sleeve 1 which is already attached to the receiving part 200 is such that the projections 27 a, 27 b are above the recesses 207 a, 207 b of the receiving part. The interlocking bushing extends slightly into the second section 14 of the first sleeve 1. In FIG. 22 the projections 27 a, 27 b have entered the recesses 207 a, 207 b fully. This is achieved, as shown in FIG. 23, by rotating the interlocking bushing 3 so that it advances in the threaded section 16 of the first sleeve 1. Because the interlocking bushing 3 abuts against the stop 21 a in the second sleeve 2, the interlocking bushing 3 pushes the second sleeve 2 forward when it moves. The second sleeve 2 can rotate relative to the bushing 3 so that the alignment between the legs 18 a, 18 b of the first sleeve 1 and the legs 25 a, 25 b of the second sleeve 2 is maintained. A further rotation of the interlocking bushing 3 presses the flat surface portion 26 a, 26 b of the front end 2 a of the second sleeve 2 onto the free flat end surfaces of the receiving part 200. Through this and the engagement of the projections 120 a, 120 b with the grooves 206 a, 206 b the first sleeve 1 is interlocked with the receiving part 200 and with the second sleeve 2 to provide a safe and strong connection between the extension device and the receiving part. In such a configuration, the insertion of the rod and locking screw can take place as well as surgical steps thereafter such as compression and distraction steps thereby using the extension device.

Turning the interlocking bushing 3 in the opposite direction releases the interlocking connection and permits to retract the projection 27 a, 27 b of the second sleeve 2 out of the recesses 207 a, 207 b. The extension device may be detached from the receiving part 200 by first removing the second sleeve 2 together with the interlocking bushing 3 and then using a tool for detaching the first sleeve such that the circumferential projection 120 a, 120 b disengage from the grooves 206 a, 206 b of the receiving part 200.

A second embodiment of the extension device is shown in FIGS. 24 to 27. All parts and portions of the second embodiment that are identical or similar to that of the first embodiment are marked with the identical reference numerals and the description thereof is not repeated. The second embodiment of the extension device differs from the first embodiment by the design of the front portion of the first sleeve and the second sleeve. Also the receiving part differs from the receiving part of the first embodiment.

The extension device according to the second embodiment comprises a first sleeve 1′, a second sleeve 2′ and the interlocking bushing 3 that is the same as in the first embodiment. As can be seen in FIG. 24, the receiving part 200′ comprises in addition to the circumferential grooves 206 a, 206 b on each leg 205 a, 205 b a coaxially extending recess 210 a, 210 b that is open towards the top end 200 a. The recesses 210 a, 210 b are provided in the outer surface of the legs 205 a, 205 b and do not completely extend through the wall of the legs 205 a, 205 b. The position of the recesses 210 a, 210 b is substantially in the center of each leg 205 a, 205 b. Furthermore, the recesses 210 a, 210 b extend through and beneath the circumferential grooves 206 a, 206 b. The contour of the recesses 210 a, 210 b is substantially rectangular, but any other shape that achieves the result of providing an abutment against a rotational motion of the first sleeve 1′ may be contemplated.

Referring to FIGS. 25 and 26, the first sleeve 1′ comprises two projections 129 a, 129 b that are extending from the front end 1 a toward the longitudinal axis c and that are positioned, sized and shaped complementary to the recesses 210 a, 210 b of the receiving part 200′. As depicted in FIG. 26, the projections 129 a, 129 b project slightly over the arc shaped end surface of each leg 18 a, 18 b of the first sleeve 1′. The engagement of the projections 129 a, 129 b with the corresponding coaxial recesses 210 a, 210 b of the receiving part 200′ generates a form fit connection that is configured to inhibit a rotational movement of the first sleeve 1′ relative to the receiving part 200′.

The second sleeve 2′ differs from the second sleeve 2 of the first embodiment in that the projections 27 a, 27 b are omitted. Hence, the front end 2 a of the second sleeve 2′ comprises only two arc shaped flat surfaces 26 a′, 26 b′ corresponding to each leg 25 a, 25 b as shown in FIG. 27.

The attachment of the extension device to the receiving part 200′ will be explained with reference to FIGS. 28 to 30 b. First, as shown in FIG. 28, the extension device according to the second embodiment is moved with its front end 1 a of the first sleeve 1′ toward the receiving part 200′ in such a manner that the legs 18 a, 18 b of the first sleeve 1′ are aligned with the legs 205 a, 205 b of the receiving part 200′. A further downward movement of the first sleeve 1′ causes an engagement of the coaxial projections 129 a, 129 b of the first sleeve 1′ with the coaxial recesses 210 a, 210 b of the receiving part 200′ as well as a snap in of the circumferential projections 120 a, 120 b into the circumferential grooves 206 a, 206 b of the receiving part 200′. Once the grooves and the recesses are in full engagement, an axial and a rotational movement of the first sleeve 1′ relative to the receiving part 200′ is inhibited. Thereafter, the second sleeve 2′ is advanced toward the receiving part 200′ by rotating the interlocking bushing 3 until the free end surfaces 26 a′, 26 b′ abut against the free end surface 200 a of the first end of the receiving part 200′ as shown in detail in FIG. 30 b. Final tightening of the interlocking bushing 3 generates a force-fit connection between the abutting surfaces of the second sleeve 2′ and the receiving part 200′ and an interlocking of the extension device relative to the receiving part. Through rotating the interlocking bushing 3 in the opposite direction the connection is released.

A modified embodiment of the design of the second sleeve is shown in FIG. 31. The second sleeve 2″ may have at its front end 2 a instead of the projection 27 a, 27 b according to the first embodiment projections 270 a, 270 b that are flush with the inner and outer surface of the legs 25 a, 25 b and have an arc-shape with a flat front surface and perpendicular side walls or inclined side walls. The projections 270 a, 270 b may be easy to manufacture in that just a portion of the leg 25 a, 25 b is cut away. A corresponding receiving part has complementary shaped recesses for engagement with the projections 270 a, 270 b.

A still further modified embodiment of the second sleeve is shown in FIG. 32. The second sleeve 2″′ comprises at its front end 2 a two recesses 290 a, 290 b on the legs 25 a, 25 b. A corresponding receiving part (not shown) comprises a complementary shape to achieve a form-fit connection.

A third embodiment of the extension device and of a system comprising the extension device and a receiving part of a polyaxial bone anchor is shown in FIGS. 34 to 38. A receiving part 200″ that is adapted to cooperate with the extension device is similar to the receiving part 200 of the first embodiment. Parts that are identical or highly similar to the corresponding parts of the first embodiment are indicated with the same reference numerals and the description thereof is not repeated. The receiving part 200″ comprises a pair of longitudinal grooves 212 a at one side of the U-shaped recess 204 and a pair of longitudinal grooves 212 b at the opposite side. The grooves of each pair of grooves 212 a, 212 b are spaced apart from each other in the circumferential direction.

Furthermore, the grooves extend from the top end 200 a in the outer wall of the receiving part on either side of the recesses 207 a, 207 b. The grooves 212 a, 212 b are open towards the top end 200 a and have a decreasing depth towards their closed end that is located at a distance from the top end 200 a. The cross-section of the longitudinal grooves 212 a, 212 b is substantially rectangular. As can be seen in particular in FIGS. 34 and 36, the longitudinal grooves 212 a, 212 b intersect the circumferentially extending grooves 206 a, 206 b. The purpose of the longitudinal grooves 212 a, 212 b is to cooperate with corresponding longitudinally extending pairs of ribs of the first sleeve in order to provide a form-fit connection.

The extension device according to the third embodiment comprises a first sleeve 1″ that is similar to the first sleeve 1 of the first embodiment. In the following, only the differences will be explained. The first sleeve 1″ comprises at positions, that correspond in a circumferential direction to the positions of the longitudinal grooves 212 a, 212 b, a pair of ribs 112 a and a pair of ribs 112 b on the legs 18 a, 18 b. The ribs 112 a, 112 b protrude inwards from the legs 18 a, 18 b and downwards from the front end 1 a and have a shape that substantially matches the shape of the grooves 212 a, 212 b. As can be seen in particular in FIG. 38, an outer surface of the ribs 112 a, 112 b tapers towards the free end, such that, when seen in a circumferential direction, the cross-section of the ribs is substantially triangular.

From each of the longitudinal ribs 112 a, 112 b a transverse thickened rib portion 123 a, 123 b extends outward in a circumferential direction. This transverse rib portion 123 a, 123 b is located at a distance from the front end 1 a that corresponds to the distance of the circumferentially extending groove 206 a, 206 b from the top end 200 a. Hence, the transverse rib portion 123 a, 123 b is at the same level as the circumferential projection 120 a, 120 b.

The second sleeve may be identical to the second sleeve 2 of the first embodiment that has the projections 27 a, 27 b for engaging the recesses 207 a, 207 b on the top end 200 a of the receiving part 200″.

By means of the longitudinal ribs 112 a, 112 b that engage the longitudinal grooves 212 a, 212 b, respectively, the strength of the connection between the head extension device and the receiving part is enhanced. The longitudinal ribs and grooves allow to apply a high torque with the extension device to the receiving part.

In use, the first sleeve 1″ is snapped over the receiving part 200″ until the form-fit connection between the circumferential projection 120 a, 120 b with the transverse portion 123 a, 123 b and the circumferential groove 206 a, 206 b is established. The longitudinal ribs 112 a, 112 b engage the longitudinal grooves 212 a, 212 b. Thereby, rotation between the head extension device and the receiving part is prevented.

It shall be noted that for the second sleeve 2 also the second sleeve 2′, 2″, 2′″ of the second embodiment may be used. The number of cooperating longitudinal ribs and grooves may vary. It should be at least one rib and one groove on each leg but can be also more than two.

The parts of the extension device are all made of a body-compatible material, such as titanium or stainless steel, a body-compatible metal alloy, for example Ti—Ni alloy, such as Nitinol, or a body-compatible plastic material, such as PEEK.

In the clinical use, as shown in FIG. 33, an extension device according to each of the embodiments described above is attached to a receiving part 200 of a bone anchor that has been inserted into a pedicle of a vertebra 501, 502, 503. The legs of the sleeves of the extension device are aligned with the legs of the receiving part such that by rotating the extension devices the channels of the receiving parts of the bone anchors can be aligned to permit the insertion of a rod. Because the connection between the receiving part and the extension device is robust and safe, an easy alignment using the extension devices is possible. Thereafter, the rod is inserted (not shown) and fixed with a locking screw that is guided through the first and second sleeve of the extension device until it can be screwed between the legs 205 a, 205 b of the receiving part. By applying an instrument to the extension device, a compression or distraction procedure can be performed using a minimally invasive technique.

Modifications of the above described embodiments may be contemplated. It shall be noted, that the shape of the engaging complementary structures of the first sleeve and the receiving part as well as the second sleeve and the receiving part can be modified and are not limited to the exact shape shown in the embodiments.

The extension device can be used with any bone anchor that comprises a receiving part, such as polyaxial bone anchor, a monoaxial bone anchor and to different shapes of receiving parts. Anchors with inner compression members or outer rings may be used. The only necessity is the engagement structure at an upper end of the receiving part that cooperates with a corresponding structure of the extension device. 

1. An extension device for a bone anchor, wherein the bone anchor comprises an anchoring section for anchoring in a bone and a receiving part connected to the anchoring section, the receiving part comprising a central axis and a channel for receiving a rod, wherein sidewalls of the channel form two free legs, the extension device comprising a first sleeve with a first sleeve axis that is coaxial with the central axis, wherein the first sleeve is configured to be detachably coupled to the receiving part and a second sleeve with a second sleeve axis, wherein the second sleeve is positioned within the first sleeve, wherein the first sleeve is configured to contact to the receiving part such as to inhibit a translational movement of the first sleeve relative to the receiving part along the central axis and wherein the second sleeve is configured to be coupled to the receiving part so as to hinder or inhibit a rotational movement of the second sleeve relative to the receiving part; characterized in that the second sleeve is connected to the first sleeve through a coupling member that is configured to advance in the first sleeve together with the second sleeve in an axial direction.
 2. The extension device of claim 1, wherein the coupling member is coupled to the first sleeve through an advancement structure that permits to advance the coupling member with the second sleeve to a given position.
 3. The extension device of claim 1, wherein the coupling member is a bushing and the advancement structure comprises threads.
 4. The extension device of claim 3, wherein the second sleeve is coupled to the first sleeve in a rigid manner.
 5. The extension device of claim 1, wherein the coupling member is coupled to the second sleeve so that it can rotate with respect to the second sleeve.
 6. The extension device of claim 1, wherein the second sleeve comprises an abutment for the coupling member.
 7. The extension device of claim 1, wherein the first sleeve comprises a protrusion and/or a recess at an inner surface thereof that is configured to engage a complementary structure at an outer surface of the receiving element to provide a form-fit connection between the first sleeve and the receiving part.
 8. The extension device of claim 7, wherein the protrusion and/or recess at the first sleeve and the complementary structure at the receiving part are shaped so as to inhibit an axial movement of the first sleeve relative to the receiving part.
 9. The extension device of claim 7, wherein the protrusion and/or recess at the first sleeve and the complementary structure at the receiving part are shaped so as to inhibit a rotational movement of the first sleeve relative to the receiving part.
 10. The extension device of claim 1, wherein the second sleeve comprises a protrusion and/or a recess at a surface thereof that is configured to engage a complementary structure at a surface of the receiving part to provide a form-fit connection between the second sleeve and the receiving part.
 11. The extension device of claim 1, wherein the second sleeve comprises a substantially flat surface portion that is configured to engage a substantially flat surface portion at the receiving part such that a friction between the surface portions is generated when the first sleeve is rotated relative to the second sleeve.
 12. The extension device of claim 1, wherein the first sleeve comprises a first end and an opposite second end and wherein the first sleeve comprises a longitudinal recess that extends in an axial direction and that is open to the second end, the width of the recess preferably being adapted to the width of the channel for the rod of the receiving part.
 13. The extension device of claim 1, wherein the second sleeve comprises a first end and an opposite second end and wherein the second sleeve comprises a longitudinal recess that extends in an axial direction and that is open to the second end, the width of recess preferably being adapted to the width of the channel for the rod of the receiving part.
 14. The extension device of claim 1, wherein the first sleeve comprises at least one longitudinally extending rib that engages a corresponding longitudinally extending groove at the receiving part so as to inhibit a rotational movement of the receiving part relative to the first sleeve.
 15. The extension device of claim 1, wherein the first sleeve comprises a circumferentially extending rib that engages a circumferentially extending groove of the receiving part in order to inhibit an axial movement of the first sleeve relative to the receiving part.
 16. A system of an extension device according to claim 1 and a bone anchor, wherein the bone anchor comprises the anchoring section for anchoring in a bone and the receiving part connected to the anchoring section, the receiving part comprising a central axis and a channel for receiving a rod, wherein sidewalls of the channel form two free legs.
 17. A spinal stabilization system for use with minimally invasive surgery comprising a system with at least two bone anchors and an extension device for each bone anchor according to claim
 1. 